1. Field of the Invention
The present invention relates to a method and device for automatic frequency control in receivers such as an FM receiver, GMSK (Gaussian Filtered Minimum Shift Keying) receiver, etc.
2. Description of the Related Art
Heretofore, such an automatic frequency control circuit as shown in FIG. 1 is known. In this circuit, a mixer 1 mixes a received signal and an output signal of a voltage-controlled local oscillator 2 to obtain an intermediate-frequency signal. The intermediate-frequency signal is converted by a frequency detector 3 to a voltage signal. The voltage signal is then output as a detector output signal. The detector output signal is also smoothed out by a time-constant circuit 4 to produce a DC voltage signal. The DC voltage signal is applied to the voltage-controlled local oscillator 2 to adjust its output frequency. That is, the conventional automatic frequency control circuit employs a feedback control system which feeds the DC voltage signal resulting from smoothing the detector output back to the voltage-controlled local oscillator 2 as an error voltage so that the difference between the received frequency and the local oscillator frequency, or the intermediate frequency may always match the center frequency of the frequency detector. (Refer to "NHK Radio FM technical textbook" Nippon Hoso Kyokai, Aug. 1, 1983, pp. 236 and 237.)
A problem with the conventional circuit is that the use of the time-constant circuit involves a time delay in smoothing the detector output and thus, when burst-like signals that are short in duration are received, the automatic frequency control circuit cannot respond to them, failing to achieve proper reception.